Two-photon absorption and effective broadband optical power limiting properties of a multi-branched chromophore containing 2,3-diarylquinoxalinyl moieties as the electron-pulling units

Article abstract of DOI:10.1016/j.tet.2009.11.109

A novel multi-branched chromophore containing 2,3-diarylquinoxalinyl units as the electron-acceptors had been synthesized and its nonlinear optical properties were characterized in the femtosecond and nanosecond regime. The experimental results show that the studied fluorophore possesses strong and wide-dispersed two-photon absorption in near infrared (NIR) region. It is demonstrated that the incorporation of 2,3-disubstituted quinoxaline moieties as a part of π-conjugation in a dye molecule could be a useful approach toward large molecular two-photon absoptivities within the studied spectral region. Effective optical-power-attenuation behaviors in the nanosecond time domain of this compound were also investigated and the results indicate that such dye molecule can be a potential material as a broadband and quick-responsive optical limiter especially when against those laser lights with longer pulses.

Full text of DOI:10.1016/j.tet.2009.11.109

Tetrahedron 66 (2010) 1375–1382
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Two-photon absorption and effective broadband optical power limiting properties
of a multi-branched chromophore containing 2,3-diarylquinoxalinyl
moieties as the electron-pulling units
*
Tzu-Chau Lin , Yu-Jhen Huang, Yong-Fu Chen, Chia-Ling Hu
Photonic Materials Research Laboratory, Department of Chemistry, National Central University, Jhong-Li 32001, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 October 2009
Received in revised form 27 November 2009
Accepted 27 November 2009
Available online 5 December 2009
A novel multi-branched chromophore containing 2,3-diarylquinoxalinyl units as the electron-acceptors
had been synthesized and its nonlinear optical properties were characterized in the femtosecond and
nanosecond regime. The experimental results show that the studied fluorophore possesses strong and
wide-dispersed two-photon absorption in near infrared (NIR) region. It is demonstrated that the in-
corporation of 2,3-disubstituted quinoxaline moieties as a part of p-conjugation in a dye molecule could be
a useful approach toward large molecular two-photon absoptivities within the studied spectral region.
Effective optical-power-attenuation behaviors in the nanosecond time domain of this compound were also
investigated and the results indicate that such dye molecule can be a potential material as a broadband and
quick-responsive optical limiter especially when against those laser lights with longer pulses.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Quinoxaline
Two-photon absorption
Optical power limiting
1. Introduction
while maintaining linear transparency over wide spectral range13
and this is a beneficial feature especially for the broadband optical
Although two-photon absorption (2PA) was theoretically pre-
dicted by Maria Go¨ppert-Mayer in 1931,1 the first experimental
observation of this nonlinear optical phenomenon was realized
about 30 years later when Kaiser and Garret observed two-photon
induced up-converted fluorescence emission from a CaF2:Eu2þ in-
organic crystal in 1961.2 With the advent of high peak-power laser
systems, numerous 2PA-based photonic and biophotonic applica-
tions have been explored and demonstrated including optical
power limiting, frequency up-converted lasing, 3-D data storage, 3-
D microfabrication, nondestructive bio-imaging and tracking, and
two-photon-assisted photodynamic therapy.3 To be of use in these
applications, organic compounds that manifest strong 2PA within
specific spectral region are consequently in great demand. The
accumulated knowledge and experience based on enormous efforts
to understand the connections between molecular structure
and 2PA property by testing and modeling various dye systems
ever since mid-1990s have revealed that large 2PA can be achieved
by judicious control of the intramolecular charge-transfer effi-
limiting applications based on 2PA. In general, fluorophores with
octupolar and/or higher-polar characters are composed of branched
or dendritic backbones. From the viewpoint of structural motif,
branched architecture not only provides an access to incorporating
several 2PA-enhancing parameters into a single molecular system but
also allows materialchemiststo possiblyoptimizea dye moleculethat
can simultaneously combine various expected characteristics for dif-
ferent specific applications. Besides, branched skeletons are potential
building blocks for constructing dendritic and/or supermolecular
structures, which are suggested as probable approaches to pursue the
fundamental limits of molecular 2PA.14 In searching for effective
strategies toward highly efficient 2PA materials, we have been in-
terested in exploring the 2PA properties of multi-branched or den-
dritic organic structures with various types of
p-conjugation
frameworks. In this paper, we report our studies of degenerate two-
photon absorption, up-converted emission and effective optical
power limiting properties of a newly synthesized multi-branched
model chromophore by using high peak power IR laser pulses work-
ing in the femtosecond and nanosecond regimes as the probing tools.
ciency and/or effective size of
p-conjugation domain within
a molecule.4–10 Among the investigated dye systems, it has been
experimentally shown that octupolar chromophores could lead to
highly efficient multi-photon absorption5d–f,i,6,7d,8a,b,f,9c,e–h,11,12
2. Results and discussion
2.1. Molecular structures and syntheses
The chemical structure and the synthetic procedure of the
studied model chromophore (1) in the present work are illustrated
* Corresponding author. Tel.: þ886 3 4227402; fax: þ886 3 4227664.
E-mail address: tclin@ncu.edu.tw (T.-C. Lin).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.11.109

1376
T.-C. Lin et al. / Tetrahedron 66 (2010) 1375–1382
in Scheme 1. The scaffold of compound 1 is constructed by placing
an electron-donating core (i.e., triphenylamine) in the center,
which is connected to three 2,3-diarylquinoxalinyl units via vinyl-
ene groups to form a six-branched dendritic structure. Recently,
electro-deficient quinoxaline moiety has been actively used as the
acceptor to produce highly efficient bipolar luminescent materials
due to its great diversity of tunable electronic properties resulting
from the manner of aromatic substitution, where the systematic
alteration is mainly carried out by incorporating electron-donating
groups to the 2,3-or 5,8-position of the quinoxaline moiety.15 Other
than the light-emitting materials, so far only limited reports on 2PA
and/or c3 materials can be found that containing quinoxalinyl
moieties.16 Therefore, it might be worthy to explore more about the
usage of quinoxaline units in design and synthesis of two-photon
active materials. Our original molecular design of this model
compound in the present work was based on a simple intention
which is to use quinoxaline moiety as an electron-withdrawing unit
and attach various electron-donating units to the 2, 3, and 6 posi-
tions of each quinoxaline moiety so that the resulting fluorophore
may have overall expanded
p-domain and possess larger multi-
polar characters. Furthermore, the pendent alkyl chains on all flu-
orenyl moieties in this model chromophore are expected to
enhance its solubility in common organic solvents, which is an-
other important parameter to be considered in the molecular de-
sign from either the aspect of experiments or applications. The
synthesis of this model compound is relatively straightforward as
outlined in Scheme 1. We have used 2-bromo-7-diphenylamino-
fluorene (2) as the starting material and followed the established
protocols for the consecutive chemical alteration to accomplish
compound (4), which was then coupled with (2) under Sonogashira
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
(b)
(a)
TMS
H
C
C
N
Br
N
C
N
C
2
3
4
C6H13
C6H13
C6H13
C6H13
(c)
O
(d)
N
N
N
N
C6H13
C6H13
O
C6H13
C6H13
5
6
N
N
(e)
(f)
NH2
NH2
C6H13
C6H13
Br
C6H13
C6H13
7
N
N
N
8
3
9
Br
Representative numbering
of C and H on fluorene &
quinoxaline units
N
N
C6H13
C6H13
f
C6H13
C6H13
e
d
N
4
13
c
N
N
b
N
C6H13
3
C6H13
N
a
12
17
16
5
2
1
TMS
C
N
C
6
C6H13
N
10
9
7
C6H13
8
11
N
15
14
N
N
N
C6H13
C6H13
N
C6H13
C6H13
C6H13
N
C6H13
C6H13
A
B
N
N
C6H13
G
H
F
C
D
N
E
1
Br
Scheme 1. Synthetic procedure for the studied model compound 1. Reagents and conditions: (a) 2 (1 equiv), PdCl2(PPh3)2 (0.06 equiv), CuI (0.1 equiv), i-Pr2NH (6 equiv), trime-
thylsilyl acetylene (1.05 equiv) in benzene, 80 ꢀC, 18 h (80%); (b) 3 (1 equiv), K2CO3 (2 equiv) in ethyl ether–methanol, rt, 4 h (94%); (c) 4 (1 equiv), 2 (1.1 equiv), Pd(PPh3)4
(0.05 equiv), CuI (0.1 equiv), i-Pr2NH (1.5 equiv) in THF, 70 ꢀC, 24 h (39%); (d) 5 (1 equiv), KMnO4 (4 equiv), NaHCO3 (1 equiv), Aliquat 336 (0.05 equiv) in CH2Cl2–H2O, rt, 12 h (71%);
(e) 6 (1 equiv), 7 (1.1 equiv) in CH3COOH, reflux, 6 h (70%); (f) Heck reaction: 8 (3.3 equiv), 9 (1 equiv), Pd(OAc)2 (0.06 equiv), P(o-tolyl)3 (0.36 equiv) in NEt3–MeCN, 110 ꢀC, 48 h
(66%). The numbering manners of carbons and hydrogens for the representative structures apply to all other compounds in this work.

T.-C. Lin et al. / Tetrahedron 66 (2010) 1375–1382
1377
reaction condition to form compound (5) in 39% yield. Compound
(5) was oxidized by KMnO4 to afford its diketone derivative (6),
which is the major synthon for the synthesis of the 2,3,6-tri-
substituted quinoxaline (8). By following the Heck reaction
method, the targeted model chromophore was accomplished in
66% yield. The detailed syntheses including the preparation of the
key intermediates (3–6, 8) and the final catalytic coupling reaction
toward the targeted model compound are described in the Exper-
imental section.
through a Shimadzu 3501 PC spectrophotometer and the 1PA-in-
duced fluorescence spectra were measured utilizing a Jobin-Yvon
FluoroMax-3 spectrometer. Compound (1) was found to show
strong and broad 1PA within the spectral range of 300–475 nm.
Under the irradiation of common UV-lamp, this model compound
also exhibits solvent dependent photoluminescence and the mea-
sured emission spectra of this model fluorophore in different sol-
vents are illustrated in the inset of Figure 1.
2.2.2. Two-photon-excited fluorescence (2PEF) emission proper-
ties. The studied model chromophore manifests intense two-pho-
ton-excited upconversion emission which can be easily observed by
naked eyes even under the illumination of an w790 nm unfocused
femtosecond laser beam. Figure 2(a) illustrates the 2PA-induced
fluorescence spectra of the model chromophore 1 in solution phase.
The sample solutions were freshly prepared at concentration of
1ꢁ10ꢂ4 M in various organic solvents for this measurement and the
excitation source utilized for this two-photon induced fluorescence
study is from a mode-locked Ti: Sapphire laser (Tsunami pumped
with a Millennia 10 W, Spectra-Physics), which delivers w80 fs
pulses with the repetition rate of 80 MHz and the beam diameter of
2 mm. The intensity level of the excitation beam was carefully
controlled in order to avoid the saturation of absorption and
photodegradation. To minimize the effect of re-absorption due to
the relatively high concentration of the sample solution used in this
measurement, we have focused the excitation beam as close as
possible to the wall of the quartz cell (5 mmꢁ5 mm cuvette) so that
only the emission from the surface of the sample was recorded. It
can be seen in Figure 2(a) that for the model chromophore the
shape and spectral position of each measured 2PA-induced
emission is basically identical to its corresponding 1PA-induced
fluorescence band shown in Figure 1. This result implies that in our
dye system the radiative relaxation processes occurred within the
studied samples are from the same final excited states regardless of
the excitation method.
2.2. Optical properties characterization
2.2.1. One-photon absorption (1PA) and fluorescence spectra meas-
urement. Linear absorption and fluorescence spectra of the studied
model compound (1) in THF solution (with concentration of
1ꢁ10ꢂ5 M) are shown in Figure 1. The 1PA spectra were recorded
2.4
2.2
Toluene
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
THF
DMF
1.2
1.0
0.8
0.6
0.4
0.2
0.0
450 500 550 600 650 700 750 800
Wavelength (nm)
300 400 500 600 700 800 900 1000 1100 1200
Wavelength (nm)
Figure 1. Linear absorption and fluorescence spectra (see the inset) of compound 1 in
solution phase (concentration was controlled at 1ꢁ10ꢂ5 M in all cases). The excitation
wavelength was fixed at 450 nm for the fluorescence spectra measurements.
The power-squared dependence of the 2PA-induced fluores-
cence intensity on the excitation intensity of the studied
(a)
Toluene
THF
λ
~ 790 nm
1.2
1.0
0.8
0.6
0.4
0.2
0.0
exc
DMF
500
550
600
650
700
750
800
Wavelength (nm)
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
4.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
(d)
(c)
(b)
4.2
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
Slope = 2.01
-0.6 -0.4 -0.2 0.0 0.2
Slope = 2.06
Slope = 2.08
-0.6 -0.4 -0.2 0.0 0.2
-0.6 -0.4 -0.2 0.0 0.2
Log (input intensity)/a.u.
Figure 2. (a) Normalized two-photon-excited upconversion spectra of fluorophore 1 in various solvent at 1ꢁ10ꢂ4 M; (b), (c) and (d) are the logarithmic plots of power-squared
dependence of the 2PA-induced fluorescence intensity on the input intensity of compound 1 in different solvents. ((b): in toluene; (c): in THF; (d): in DMF).

1378
T.-C. Lin et al. / Tetrahedron 66 (2010) 1375–1382
fluorophore was also examined. Figure 2(b)–(d) are logarithmic
plots of the measured data and the results confirm that 2PA process
is responsible for the observed up-converted fluorescence
emissions in all cases.
We have used fluorescein (w80
m
M in pH¼11 NaOH solution) as the
standard for this experiment.17 Figure 4 shows the measured de-
generate two-photon absorption spectra of this model compound
in THF (at the concentration of 1ꢁ10ꢂ4 M) and the combined
photophysical data are summarized in Table 1. It is notable that this
model chromophore exhibits strong 2PA (d2ꢃ650 GM) from 700–
850 nm with an absorption maximum located around 830 nm
The temporal behavior and lifetime of 1PA- and 2PA-induced
fluorescence of the same sample solutions were also probed based
on the time-correlated single photon counting (TCSPC) technique
using a highly sensitive photomultiplier equipped with accumulat-
ing real-time processor as the detection system (PMA-182 and
TimeHarp 200, PicoQuant). The same Ti: sapphire laser system (vide
supra) was employed for this experiment. The measured fluores-
cence decay curves of the studied compound solutions are depicted
in Figure 3 on a 10-ns full scale for one- and two-photon excitation
(in the femtosecond regime). Theoretical fitting of each decay curve
(d2w3100 GM).
2.3. Discussion of results
From the measured photophysical properties of the studied
model compound in the present work, some features are
notable:
(b)
(a)
Compound 1 in toluene
Compound 1 in THF
2.0
Two-photon excitation (at ~790 nm)
Exponential fitting parameter
τ = 3.1 ns
Two-photon excitation (at ~790 nm)
Exponential fitting parameter
τ = 1.6 ns
1.5
1.0
One-photon excitation (at ~395 nm)
Exponential fitting parameter
τ = 3.1 ns
One-photon excitation (at ~395 nm)
Exponential fitting parameter
τ = 1.6 ns
0.5
0.0
0
2
4
6
8
10
0
2
4
6
8
10
Time (ns)
Time (ns)
Figure 3. Normalized fluorescence decay curves of the studied model compound solutions excited by one-photon absorption (at w395 nm) and two-photon absorption
(at w790 nm): (a) in toluene; (b) in THF.
Table 1
Photophysical properties of compound 1 in toluene and THF
1PAꢂFL
Compound
l1mPaAx=nma,b
log 3max
l
=nmb,d
FF
s1PAꢂFL=nsc,f
2PA-related properties
c
c,e
max
2PAꢂFL
Toluene
450
THF
452
Toluene
505
THF
570
dm2 ax=GMc,g
w3100
l
=nmc,h
s2PAꢂFL=nsc,i
max
1
5.25
0.56
3.1
571
3.1
a
One-photon absorption maximum.
The concentration was 1ꢁ10ꢂ5 M.
In THF.
Spectral position of 1PA-induced fluorescence emission maximum.
Fluorescence quantum efficiency.
b
c
d
e
f
1PA-induced fluorescence lifetime.
The concentration was 1ꢁ10ꢂ4 M for 2PEF measurements; dm2 ax :the maximum 2PA cross-section value (with experimental error¼ꢄ15%); 1GM¼1ꢁ10ꢂ50 cm4 s/photon-
g
molecule.
h
Spectral position of 2PA-induced fluorescence emission maximum.
2PA-induced fluorescence lifetime.
i
(1) If one compares the measured 1PA and 2PA spectra of this
compound in THF solution (See Figs. 1 and 2), it can be readily
found that this model chromophore exhibits detectable 2PA at
wavelengths equal to twice of those wavelengths in ultravio-
let–visible (UV–vis) region where it possesses strong 1PA. This
indicates that many excited states of this model dye molecule
are energetically accessible by both one-photon and two-
photon transitions. Besides, the widely dispersed two-photon
activities in near-IR spectral regime suggests that this model
chromophore could be a potential candidate as a broadband
optical power limiter when against ultra-short laser pulses.
by single-exponential dependence has revealed that for each chro-
mophore solution identical 1PA-/2PA-induced fluorescence lifetime
were observed and this phenomenon is independent of the excita-
tion process. The measured 1PA-/2PA-induced fluorescence lifetime
values for each model compound solution are listed in Table 1.
2.2.3. Degenerate two-photon absorption spectra measurement. The
2PA behaviors of the studied dye molecule as a function of wave-
length was explored in the spectral region of 700–850 nm by the
degenerate two-photon-excited fluorescence (2PEF) technique
based on a set-up very similar to that reported by Xu and Webb.17

T.-C. Lin et al. / Tetrahedron 66 (2010) 1375–1382
1379
optical-power-attenuators against the laser pulses working in
nanosecond time domain.3c,22
3500
3000
2500
2000
1500
1000
500
Measured data for compound 1
It is noted that the excited-state lifetime of the studied model
chromophore in the present work are in the nanosecond range,
which indicates that significant excited-state population may be
retained and may consequently possess excited-state absorption
during the excitation by longer laser pulses. In order to study the
effective powerlimitingperformance of this modelchromophore(1),
we have utilized nanosecond laser pulses for this investigation. In
our experiment, the nonlinear absorbing medium was a 1-cm path-
length solution of 1 in THF with concentration of 0.02 M. As
an excitation laser light source, a tunable nanosecond laser system
(an integrated Q-switched Nd:YAG laser and OPO: NT 342/3 from
Ekspla) was employed to provide w6 ns laser pulses with controlled
average pulse energy of w1 mJ and repetition rate of 10 Hz for this
study. The laser beam was slightly focused onto the center of the
sample solution in order to obtain a nearly uniform laser beam ra-
dius within the whole cell path-length. The transmitted laser beam
from the sample cell was detected by an optical power (energy)
meter with a large detection area of w25 mm in diameter. Figure 5
shows the measured average transmittance in the spectral range of
700–1000 nm. Although the entire dispersion of ESA as a function of
wavelength (i.e., ESA spectra) is not yet revealed at current stage, it is
noted that underourexperimental conditions this model compound
can attenuate nanosecond laser pulses within a broad spectral re-
gion especially around 800 nm. The attenuation of the incident laser
pulses based on this sample solution can be presented in another
way as shown in the inset of Figure 5. In this experiment, the local
intensity within this sample solution was tuned by a set of neutral-
density filter. Three representative wavelengths have been selected
for this demonstration of power restriction efficiency. One can see
that compound 1 displays the best power restriction properties at
800 nm. This initial finding suggests the potentiality of using this
model fluorophore for broadband power-suppressing-related ap-
plications in the nanosecond regime.
0
700 720 740 760 780 800 820 840 860
Wavelength (nm)
Figure 4. Measured degenerate two-photon absorption spectra of model chromo-
phore 1 by 2PEF method in THF solution at 1ꢁ10ꢂ4 M (with experimental error
wꢄ15%).
(2) Based on the fact that the measured fluorescence emission
spectra (shown in Figs. 1 and 2) and the fluorescence lifetimes
(depicted in Fig. 3) for each sample solution are nearly the same
for one- and two-photon excitations, one may conclude that in
these cases, each fluorescence emissions is predominantly
from the same singlet state of this model chromophore.18 On
the other hand, the medium-high quantum yield observed
from compound 1 also indicates that it could be an efficient
frequency upconverter for the biophotonic applications such as
two-photon-excited fluorescence microscopy.
(3) The excited-state lifetime of this model compound is found to
be solvent dependent and can be tuned when dissolved in
different organic solvents with various polarity. (e.g.,
in toluene and 1w3.1 in THF). This feature is particularly de-
s1w1.6 ns
s
sirable with regards to optical power limiting applications in
the nanosecond regime based on 2PA-induced excited-state
absorption (2PA-induced ESA) since the effective three-photon
absorption coefficient increases significantly with the excited-
state lifetime.19
1.0
0.9
0.8
0.7
3. Effective optical power limiting behaviors in nanosecond
regime
measured data@
measured data@
measured data@
~720nm
~800nm
~900nm
2.0
1.5
1.0
0.5
0.0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
It is well known that an ideal optical-limiter is expected to show
an intensity-dependent transmission feature so that the output
intensity always below a certain maximum value, which makes
optical-limiters useful for protecting human eyes and sensors
against damaging sources of light. Additionally, this power limiting
phenomenon is also important for optical dynamic range com-
pression and noise suppression in signal processing as well as
nonlinear ultra-fast filtering/reshaping of optical fiber signals.
Compared to other mechanisms of optical power limiting such as
reverse saturable absorption and nonlinear scattering,20 the 2PA
mechanism provides several promising features includes: (1) high
initial transmission (ca.w100%) for weak optical signals; (2) quick
response to a change of the intensity or peak power of the incident
optical signal; and (3) retention of optical quality of the input beam
after passing through the nonlinear absorption medium.
0.0
0.5
1.0
1.5
2.0
Input Energy(mJ)
700
750
800
850
900
950
1000
Wavelength(nm)
Figure 5. Measured transmission spectrum based on a 1-cm path solution sample of
compound 1 in THF at 0.02 M. The energy of the input laser pulses was controlled at
w1m J for every data point. Inset: Measured optical-power-attenuation curves at
various wavelengths based on the same sample solution.
Recently, many researchers have posited that the intensity-de-
pendent 2PA-induced excited-state absorption plays an essential
role for the observed large 2PA in various organic systems under
the irradiation of nanosecond laser pulses,21 and for this reason the
term ‘effective 2PA’ is used to describe the apparent 2PA parameter
measured in nanosecond time domain.9b,21bFrom the viewpoint of
application, any medium that exhibits strong apparent nonlinear
absorption covering wide spectral range could be very useful
4. Conclusion
We have synthesized a novel octupolar chromophore with 2,3-
diarylquinoxalinyl units as the molecular termini and characterized
its nonlinear optical properties in the femtosecond and nanosecond
regime. The experimental results show that the subject compound
manifests wide-dispersed and strong two-photon absorption in
near infrared region. It is demonstrated that the incorporation of

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T.-C. Lin et al. / Tetrahedron 66 (2010) 1375–1382
2,3-disubstituted quinoxaline moieties as a part of
p-framework in
22.54 (Ce), 14.04 (Cf), 0.08 (–Si(CH3)3). Anal. calcd for C42H51NSi: C,
a dye molecule could provide an access toward large molecular
two-photon absoptivities within the studied spectral region. Ef-
fective optical-power-attenuation behaviors in the nanosecond
time domain of this model chromophore were also investigated
and the results suggest that such dye compound can be a potential
material as a broad-band and agile optical limiter especially when
against those laser lights with longer pulses.
84.36; H, 8.60; N, 2.34; Si, 4.70; found: C, 84.45; H, 8.55; N, 2.35.
5.3.2. 7-Ethynyl-9,9-dihexyl-N,N-diphenyl-9H-fluoren-2-amine
(4). A mixture of compound 3 (4.33 g; 0.00724 mol), ethyl ether
(20 mL), potassium carbonate (2 g; 0.014 mol), and methanol
(20 mL) was stirred at room temperature for 4 h. Then, w50 mL of
saturated NH4Cl solution was added into the reaction mixture and
the resultant solution was stirred at room temperature for 0.5 h.
The above solution was then extracted with ethyl acetate
(30 mLꢁ3) and then dried over MgSO4(S). After removing the sol-
vent, the crude product was purified through column chromatog-
raphy on silica gel using hexane as the eluent to give the final
purified product as pale-yellow oil with yield of w94% (3.58 g). 1H
5. Experimental
5.1. General
All commercially available reagents for the preparation of the
intermediates and targeted chromophores were purchased from
Aldrich Chemical Co. and were used as received, unless stated
otherwise. 1H NMR and 13C NMR spectra were recorded either at
200 or 300 MHz spectrometers and referenced to TMS or residual
CHCl3. High-resolution mass spectroscopy (HRMS) was conducted
by using a Waters LCT ESI-TOF mass spectrometer. MALDI-TOF MS
spectrum was obtained on a Voyager DE-PRO mass spectrometer
(Applied Biosystem, Houston, USA). Elemental analysis was per-
formed utilizing a Perkin Elmer series II CHNS/O Analyzer 2400.
NMR(300 MHz, CDCl3) d: 7.60–7.57 (m, 2H, ArH), 7.52–7.49 (m, 2H,
ArH), 7.33–7.28 (dd, 4H, H3), 7.21–7.18 (m, 5H, ArH), 7.11–7.04 (m,
3H, ArH), 3.14 (s, Ar–C^C–H, 1H), 2.00–1.84 (m, 4H, Ha), 1.23–1.12
(m, 12H, Hb, Hc, Hd), 0.89–0.84 (t, 6H, Hf), 0.74–0.72 (m, 4H, He); 13
C
NMR (75 MHz, CDCl3, tentative assignments based on calculated
values) : 152.42 (C16), 150.51 (C17), 147.81 (C1), 147.65 (C6), 141.68
d
(C15), 135.19 (C14), 131.17 (C12), 129.16 (C3), 126.25 (C10), 123.93 (C2),
123.29 (C8), 122.64 (C4), 120.75 (C7, C9), 119.28 (C5), 118.89 (C11),
84.80 (Ar–C^C–H), 76.83, (Ar–C^C–H), 55.01 (C13), 40.13 (Ca),
31.47 (Cb), 29.55 (Cc), 23.67 (Cd), 22.52 (Ce), 14.02 (Cf). HRMS-
FAB(m/z): Mþ calcd for C39H43N, 525.3396; found, 525.3402. Anal.
calcd for C39H43N: C, 89.09; H, 8.24; N, 2.66; found: C, 89.12; H,
8.26; N, 2.62.
5.2. Photophysical methods
All the linear optical properties of the subject model compound
were measured by corresponding spectrometers and the detailed
experimental conditions as well as the optical set-ups for nonlinear
optical property investigations are described in the Supplementary
Data.
5.3.3. 7-(2-(7-(Diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl)ethynyl)-
9,9-dihexyl-N,N-diphenyl-9H-fluoren-2-amine (5). To a solution of
compound 2 (3.64 g; 0.00627 mol) and 4 (3 g; 0.0057 mol) in THF
(35 mL) was added of Pd(PPh3)4 (0.33 g; 0.285 mmol), CuI
(0.1087 g; 0.57 mmol), and i-Pr2NH (0.87 g; 0.00856 mol) was
stirred at 70 ꢀC under N2 for 24 h. After cooling to the room tem-
perature, w50 mL of saturated NH4Cl solution was added into the
reaction mixture and the resultant solution was stirred at room
temperature for 0.5 h. The above solution was then extracted with
ethyl acetate (30 mLꢁ3) and then dried over MgSO4(S). After
removing the solvent, the crude product was purified by column
chromatography on silica gel using ethyl acetate–hexane (1:60) as
the eluent and recrystallized from methanol/hexane. The purified
product was obtained as yellow powder with yield of w39%
5.3. Synthesis
In Scheme 1, compounds 2 and 9 were synthesized by following
the established literature processes23,24 with overall yields of w45%
for compound 2 and w35% for compound 9. The synthesis of model
chromophore 1 was accomplished via the well-known Heck cou-
pling reaction using compounds 8 and 9 as the major synthons and
was obtained in the yield of w66%. The experimental details for the
syntheses of these compounds are presented as the following:
(2.29 g). 1H NMR(300 MHz, CDCl3)
d: 7.60–7.49 (m, 8H, ArH), 7.27–
5.3.1. 9,9-Dihexyl-N,N-diphenyl-7-(2-(trimethylsilyl)ethynyl)-9H-
fluoren-2-amine (3). To a solution of compound 2 (6 g; 0.0103 mol)
in benzene (45 mL) was added of PdCl2(PPh3)2 (0.435 g; 0.62 mmol),
CuI (0.197 g; 1.033 mmol), i-Pr2NH (6.273 g; 0.062 mol), and tri-
methylsilyl acetylene (1.066 g; 0.01085 mol) was stirred at 80 ꢀC
under Ar atmosphere for 18 h. After cooling to the room tempera-
ture, w50 mL of saturated NH4Cl solution was added into the re-
action mixture and the resultant solution was stirred at room
temperature for 0.5 h. The above solution was then extracted with
ethyl acetate (30 mLꢁ3) and then dried over MgSO4(S). After re-
moving the solvent, the crude product was purified through col-
umn chromatography on silica gel using hexane as the eluent to
give the final purified product as pale-yellow oil with yield of w80%
7.22 (dd, 8H, H3), 7.13–7.11 (d, 10H, ArH), 7.04–6.99 (m, 6H, ArH),
1.95–1.78 (m, 8H, Ha), 1.16–1.06 (m, 24H, Hb, Hc, Hd), 0.82–0.77 (t,
12H, Hf), 0.65 (br, m, 8H, He); 13C NMR (75 MHz, CDCl3, tentative
assignments based on calculated values) d: 152.43 (C16), 150.63
(C17), 147.86 (C1), 147.49 (C6), 141.10 (C15), 135.51 (C14), 130.60 (C12),
129.16 (C3), 125.69 (C10), 123.88 (C2), 123.40 (C8), 122.59 (C4), 120.64
(C7, C9), 119.01 (C5, C11), 90.49 (internal acetylene carbons), 55.08
(C13), 40.28 (Ca), 31.54 (Cb), 29.63 (Cc), 23.73 (Cd), 22.57 (Ce), 14.04
(Cf); HRMS-FAB(m/z): Mþ calcd for C76H84N2, 1024.6635; found,
1024.6639. Anal. calcd for C76H84N2: C, 89.01; H, 8.26; N, 2.73;
found: C, 88.98; H, 8.25; N, 2.77.
5.3.4. 1,2-Bis(7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl)ethane-
1,2-dione (6). To a solution of compound 5 (2.25 g; 0.00219 mol) in
CH2Cl2 (20 mL) was added KMnO4 (1.387 g; 0.00877 mol),
NaHCO3(0.184 g; 0.00219 mol), Aliquat 336(0.045 g; 0.1097 mmol),
and H2O (10 mL) was stirred at room temperature for 12 h. Then,
w50 mL of saturated NaHSO3 and 1 N HCl solutions were added into
the reaction mixture. The above solution was then extracted with
dichloromethane (30 mLꢁ3) and then dried over MgSO4(S). After
removing the solvent, the crude product was purified by column
chromatography on silica gel using dichloromethane–hexane (1:4) to
give the final purified product as saffron-yellow powder with yield of
(4.94 g). 1H NMR(300 MHz, CDCl3)
d: 7.59–7.55 (m, 2H, ArH), 7.51–
7.48 (d, 2H, ArH), 7.32–7.27 (dd, 4H, H3), 7.20–7.17 (m, 5H, ArH),
7.09–7.03 (m, 3H, ArH), 1.93–1.90 (t, Ha, 4H), 1.20–1.12 (m, Hb, Hc, Hd
12H), 0.89–0.85 (m, 6H, Hf), 0.71 (br, m, 4H, He), 0.35 (s, –Si(CH3)3,
9H); 13C NMR (75 MHz, CDCl3, tentative assignments based on
calculated values) d: 152.42 (C16), 150.41 (C17), 147.83 (C1), 147.58
(C6), 141.45 (C15), 135.37 (C14), 131.18 (C12), 129.14 (C3), 126.02 (C10),
123.88 (C2), 123.35 (C8), 122.62 (C4), 120.72 (C9), 120.34 (C7), 118.95
(C5), 118.82 (C11), 106.42 (Ar–C^C–Si(CH3)3), 93.55 (Ar–C^C–
Si(CH3)3), 55.03 (C13), 40.20 (Ca), 31.51 (Cb), 29.58 (Cc), 23.67 (Cd),

T.-C. Lin et al. / Tetrahedron 66 (2010) 1375–1382
1381
w71%(1.64 g).1H NMR(300 MHz, CDCl3)
d
: 8.04 (s, 2H, H12), 7.88–7.85
123.37 (C11), 122.51 (C4), 120.83 (C5), 119.78 (C7), 119.07 (C6 ), 55.04
(C13), 40.09 (Ca), 31.52 (Cb), 29.57 (Cc), 23.83 (Cd), 22.60 (Ce), 14.08
(Cf); MALDI-TOF MS: [MþH]þ calcd for C270H279N13, 3707.1914;
found, 3707.1974. Anal. calcd for C270H279N13: C, 87.50; H, 7.59; N,
4.91; found: C, 87.45; H, 7.62; N, 4.93.
0
(d, J¼7.8 Hz, 2H, H9), 7.65–7.62 (d, J¼7.8 Hz, 2H, H10), 7.61–7.58 (d, 2H,
H8), 7.29–7.24 (dd, 8H, H3), 7.15–7.11 (m, 10H, ArH), 7.07–7.01 (m, 6H,
ArH),1.98–1.79(m, 8H, Ha),1.15–1.04(m, 24H, Hb, Hc, Hd), 0.82–0.77(t,
12H, Hf), 0.64 (br, m, 8H, He); 13C NMR (75 MHz, CDCl3, tentative
assignments based on calculated values) d: 195.15 (carbonyl carbon),
153.83 (C16), 151.27 (C17), 149.02 (C15), 147.79 (C1), 147.55 (C6), 133.78
(C14), 130.92 (C12), 129.27 (C3), 124.42 (C2), 123.16 (C4), 122.70 (C8),
121.82 (C10, C11), 118.84 (C5), 118.02 (C7), 55.27 (C13), 39.91 (Ca), 31.44
(Cb), 29.49 (Cc), 23.73 (Cd), 22.51 (Ce), 14.01 (Cf). HRMS-FAB(m/z): Mþ
calcd for C76H84N2O2, 1056.6533; found, 1056.6627. Anal. calcd for
C76H84N2O2: C, 86.32; H, 8.01; N, 2.65; O, 3.03; found: C, 86.25; H,
8.06; N, 2.68.
Acknowledgements
We acknowledge the financial support from National Science
Council (NSC), Taiwan. We also thank Dr. Mei-Chun Tseng in the
Institute of Chemistry of Academia Sinica, Taiwan for her kind help
in MALDI-TOF spectroscopy measurements.
Supplementary data
5.3.5. 7-(6-Bromo-3-(7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-
yl)quinoxalin-2-yl)-9,9-dihexyl-N,N-diphenyl-9H-fluoren-2-amine
(8). A mixture of compound 6 (1.2 g; 0.001134 mol) and 7 (0.233 g;
0.00125 mol) in CH3COOH (30 mL) was refluxed under N2 atmo-
sphere for 6 h. After cooling to the room temperature, w50 mL of
H2O was added to the reaction mixture. After filtration, the crude
solid product was collected and recrystallized from methanol. The
purified product was obtained as green-brownish powder with
Supplementary data associated with this article can be found in
online version at doi:10.1016/j.tet.2009.11.109.
References and notes
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yield of w88% (1.21 g).1H NMR (300 MHz, CDCl3)
d: 8.37 (s, 1H, HC),
8.05–8.02 (d, J¼9 Hz, 1H, HF), 7.83–7.79 (d, J¼9 Hz, 1H, HE), 7.58–
7.50 (m, 8H, ArH), 7.26–7.21 (m, 8H, H3), 7.12–7.08 (m, 10H, ArH),
7.03–6.98 (m, 6H, ArH), 1.74–1.72 (m, 8H, Ha), 1.10–1.02 (m, 24H, Hb,
Hc, Hd), 0.80–0.76 (t, 12H, He), 0.64–0.62(m, 8H, Hf); 13C NMR
(75 MHz, CDCl3, tentative assignments based on calculated values)
d: 154.78 (CA), 154.30 (CB), 152.63 (C16), 150.62 (C17), 147.89 (C1),
147.57 (C14), 141.95 (CH), 141.88 (CG), 141.66 (C6), 139.82 (C15), 136.95
(CF), 136.85 (CC), 135.33 (CE), 133.04 (C12), 131.36 (C8), 130.39 (C9),
129.15 (C3), 124.33 (C10), 123.90 (C2), 123.34 (C11), 122.59 (C4),
120.89 (C5), 119.02 (C7), 118.89 (CD), 55.09 (C13), 40.09 (Ca), 31.53
(Cb), 29.57 (Cc), 23.83 (Cd), 22.60 (Ce), 14.07 (Cf); HRMS-FAB(m/z):
Mþ calcd for C82H87BrN4, 1206.6114; found, 1206.6104. Anal. calcd
for C82H87BrN4: C, 81.50; H, 7.26; Br, 6.61; N, 4.64; found: C, 81.52;
H, 7.23; N, 4.65.
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5.3.6. 7-(6-((E)-2-(4-(Bis(4-((E)-2-(2,3-bis(7-(diphenylamino)-9,9-di-
hexyl-9H-fluoren-2-yl)quinoxalin-6-yl)vinyl)phenyl)amino)phe-
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of compound 8 (1.1 g; 0.918 mmol), tris(4-vinylphenyl)amine (9,
0.09 g; 0.278 mmol), Pd(OAc)2 (0.0037 g; 0.0167 mmol), P(o-tolyl)3
(0.03 g; 0.1 mmol), triethylamine (5 mL), and acetonitrile (10 mL)
was loaded into a heavy-wall high pressure reaction tube equipped
with a magnetic stirrer and a rigid Teflon cap. The reaction mixture
was heated up to 110 ꢀC under N2 atmosphere and kept at this
temperature by means of an oil bath or a heating mantle for 48 h.
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acetate¼10/1). 1H NMR(300 MHz, CDCl3)
d: 8.21 (s, 3H, HC), 8.17–
8.14 (d, J¼9 Hz, 3H, HF), 8.01–7.98 (d, J¼9 Hz, 3H, HE), 7.55–7.50 (m,
36H, ArH on fluorene and central triphenylamine units), 7.39–7.34
(d, J¼15 Hz, 3H, ethylene protons), 7.29–7.24 (d, J¼15 Hz, 3H, eth-
ylene protons), 7.26–7.19 (m, 24H, H3), 7.12–7.07 (m, 30H, ArH),
7.03–6.98 (m, 18H, ArH), 1.73 (br, m, 24H, Ha), 1.13–1.02 (m, 72H, Hb,
Hc, Hd), 0.80–0.76 (t, 36H, Hf), 0.64 (br, m, 24H, He); 13C NMR
(75 MHz, CDCl3, tentative assignments based on calculated values)
d: 154.36 (CA), 153.31 (CB), 152.39 (C16), 150.52 (C17), 148.30 (C1),
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